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<h1 id="-1-">第1章：加载和执行</h1>
<h2 id="-">脚本位置</h2>
<ul>
<li>优化JS的首要规则：将脚本放在底部。</li></ul>
<h2 id="-">组织脚本</h2>
<ul>
<li>不要把内嵌脚本紧跟在<code>&lt;link&gt;</code>标签后面。</li></ul>
<h2 id="-">无阻塞的脚本</h2>
<h3 id="-">延迟的脚本</h3>
<p>任何带有<code>defer</code>属性的<code>&lt;script&gt;</code>元素在DOM完成加载之前都不会被执行。</p>
<h2 id="-">动态脚本元素</h2>
<pre><code>var script = document.createElement(&#39;script&#39;);
script.src = &quot;file.js&quot;;
document.getElementsByTagName(&quot;head&quot;)[0].appendChild(script);
</code></pre><p>浏览器那去在<code>&lt;script&gt;</code>元素接收完成时触发一个<code>load</code>事件，可以通过侦听此事件来获得脚本加载完成时的状态。</p>
<pre><code>var script = document.createElement(&#39;script&#39;);
script.onload = function () {
    console.log(&quot;Script.loaded!&quot;);
};
script.src = &quot;file.js&quot;;
document.getElementsByTagName(&quot;head&quot;)[0].appendChild(script);
//hello
//Script loaded!
</code></pre><p>IE支持另一种实现方式，它会触发一个<code>readystatechange</code>事件。</p>
<p><code>&lt;script&gt;</code>元素提供一个<code>readyState</code>属性，它的值在外链文件的下载过程的不同阶段会发生变化，该属性有<code>5</code>中取值：</p>
<ol>
<li><code>uninitialized</code> 初始状态</li><li><code>loading</code> 开始下载</li><li><code>loaded</code> 下载完成</li><li><code>interactive</code> 数据完成下载但尚不可用</li><li><code>complete</code> 所有数据已准备就绪</li></ol>
<p>使用<code>readystatechange</code>事件最靠谱的方式是同时检查这两种状态，只要其中任何一个触发，就删除事件处理器（确保事件不会处理两次）。</p>
<pre><code>var script = document.createElement(&#39;script&#39;);
script.onreadystatechange = function () {
    if (script.readyState === &quot;loaded&quot; || script.readyState === &quot;complete&quot;) {
        script.onreadystatechange = null;
        console.log(&quot;script loaded!&quot;);
    }
};
script.src = &quot;file.js&quot;;
document.getElementsByTagName(&quot;head&quot;)[0].appendChild(script);
//hello
</code></pre><p>使用单一的方法动态加载JS文件，下面的函数封装了标准及IE特有的实现方法：</p>
<p><p data-height="365" data-theme-id="0" data-slug-hash="qKMMYr" data-default-tab="js" data-user="whjin" data-embed-version="2" data-pen-title="onreadystatechange" class="codepen">See the Pen <a href="https://codepen.io/whjin/pen/qKMMYr/">onreadystatechange</a> by whjin (<a href="https://codepen.io/whjin">@whjin</a>) on <a href="https://codepen.io">CodePen</a>.</p></p>
<script async src="https://static.codepen.io/assets/embed/ei.js"></script>

<p>动态脚本加载凭借它在跨浏览器兼容性和易用的优势，成为最通用的无阻塞加载解决方案。</p>
<h2 id="-xmlhttprequest-"><code>XMLHTTPRequest</code>脚本注入</h2>
<p>另一种无阻塞加载脚本的方法是使用<code>XHR</code>对象获取脚本并注入页面中。此技术先创建一个XHR对象，然后用它下载JS文件，最后通过创建动态<code>&lt;script&gt;</code>元素将代码注入页面。</p>
<pre><code>var xhr = new XMLHttpRequest();
xhr.open(&quot;get&quot;, &quot;file.js&quot;, true);
xhr.onreadystatechange = function () {
    if (xhr.readyState === 4) {
        if (xhr.status &gt;= 200 &amp;&amp; xhr.status &lt; 300 || xhr.status === 304) {
            var script = document.createElement(&quot;script&quot;);
            script.text = xhr.responseText;
            document.body.appendChild(script);
        }
    }
};
</code></pre><p>这种方法的主要局限性是JS文件必须与所请求的页面处于相同的域，所以JS文件不能从CDN下载。因此，大型Web应用通常不会采用XHR脚本注入技术。</p>
<h2 id="-">推荐的无阻塞模式</h2>
<p>向页面中添加大量JS的推荐做法只需两步：先添加动态加载所需的代码，然后加载初始化页面所需的剩下的代码。</p>
<p>举个例子:</p>
<pre><code>&lt;script src=&quot;loader.js&quot;&gt;&lt;/script&gt;
&lt;script&gt;
    loadScript(&quot;file.js&quot;, function () {
        Application.init();
    })
&lt;/script&gt;
</code></pre><p>或者直接把函数直接嵌入页面，从而避免多产生一次HTTP请求。</p>
<h2 id="yui3-">YUI3的方式</h2>
<p>使用DOM工具包，只需在YUI的<code>use()</code>方法中声明<code>dom</code>，并且提供回调函数即可：</p>
<pre><code>YUI.use(&quot;dom&quot;, function (Y) {
    Y.DOM.addClass(document.body, &quot;loaded&quot;);
});
</code></pre><h2 id="lazyload-">LazyLoad类库</h2>
<p>更为通用的延迟加载工具：<strong><a href="https://github.com/tuupola/jquery_lazyload">LazyLoad</a></strong>。LazyLoad是<code>loadScript()</code>函数的增强版。</p>
<pre><code>LazyLoad.js(&quot;file.js&quot;, function () {
    Application.init();
});
</code></pre><p>LazyLoad支持下载多个JS文件，并能保证在所有浏览器中都以正确的顺序执行。要加载多个JS文件，只需给<code>LazyLoad.js()</code>方法传入一个URL数组：</p>
<pre><code>LazyLoad.js([&quot;file1.js&quot;, &quot;file2.js&quot;], function () {
    Application.init();
});
</code></pre><h2 id="labjs">LABjs</h2>
<p><code>$LAB.script()</code>方法用来定义需要下载的JS文件，<code>$LAB.wait()</code>用来指定文件下载并执行完毕后所调用的函数。LABjs鼓励链式调用，因此每个方法都会返回一个<code>$LAB</code>对象的引用。要下载多个JS文件，只需链式调用另一个<code>$LAB.script()</code>方法：</p>
<pre><code>$LAB.script(&#39;file1.js&#39;)
    .script(&#39;file2.js&#39;)
    .wait(function () {
        Application.init();
    });
</code></pre><p>LABjs允许使用<code>wait()</code>方法来指定哪些文件需要等待其他文件，为确保第一个文件最先执行，必须在第一个<code>script()</code>方法后面调用<code>wait()</code>。</p>
<h2 id="-">小结</h2>
<p>减少JS对性能的影响：</p>
<ul>
<li><code>&lt;/body&gt;</code>标签之前将所有的<code>&lt;script&gt;</code>标签放到页面底部。确保在脚本执行前页面已经完成渲染。</li><li>合并脚本。</li><li>有多种无阻塞下载JS文件的方法：<ul>
<li>使用<code>&lt;script&gt;</code>标签的<code>defer</code>属性</li><li>使用动态创建<code>&lt;script&gt;</code>元素来下载并执行代码</li><li>使用XHR对象下载JS代码并注入页面中</li></ul>
</li></ul>

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